Field of the Invention
The invention lies in the communications and digital signal processing fields. More specifically, the invention relates to a turbo decoder for decoding a data signal that is error-protection-coded with a turbo code at the transmitter end, transmitted via a disturbed channel and detected in a receiver. The detected data signal is decoded iteratively in a computing loop programmed to perform the following computing steps: first symbol estimation of an input data signal, taking into consideration a first partly redundant data signal and a feedback data signal; interleaving of the data signal calculated in the first symbol estimation; second symbol estimation of the interleaved data signal, taking into consideration a second partly redundant data signal; and deinterleaving the data signal calculated in the second symbol estimation, for determining the feedback data signal. The computing loop is passed a plurality of times. The invention also pertains to a method for turbo decoding a data signal that is error-protection-coded at the transmitter end, transmitted via a disturbed channel, and detected in the receiver.
A basic problem in telecommunication consists in transmitting a data signal as free of errors as possible via a disturbed channel. To achieve this, the data signal to be transmitted is subjected to channel coding. The signal to be transmitted is adapted to the characteristics of the transmission channel by means of the channel coding. A multiplicity of different channel coding methods are used. The principle of channel coding (which is also called error protection coding) is based on achieving an effective error protection by selective introduction of redundancy into the signal to be transmitted.
Particular problems in conjunction with the signal transmission arise in mobile radio systems. This is attributable to the fact, on the one hand, that in mobile radio systems, decidedly severe signal distortions occur at the receiver due to time-variant multiple reflections, common-channel interference, Doppler shift and noise. On the other hand, the receiver of a mobile station only has limited power and hardware resources for reasons of space and cost. In mobile radio engineering, therefore, it is attempted to achieve a reconstruction of the transmitted signal with the least possible error rate as hardware-efficiently as possible (i.e. saving space and cost) with the least possible expenditure on signal processing (preserving the power resource).
It can be seen that, as a rule, these requirements cannot be optimized at the same time but that a meaningful compromise between these requirements must be created by selective software/hardware design of the receiver.
A modern method of error protection coding is the so-called “turbo coding”. In turbo coding, so-called “turbo codes” are used. Turbo codes are binary recursive convolution codes that are concatenated in parallel. Turbo codes represent a decidedly effective form of error protection coding, particularly in the transmission of large data blocks.
In the receiver, a turbo decoder is used for decoding turbo codes. Such a decoder is described on pages 343 to 368, particularly FIG. E.2, of the book “Analyse und Entwurf digitaler Mobilfunksysteme” [Analysis and design of digital mobile radio systems] by P. Jung, Stuttgart, B. G. Teubner, 1997. That turbo decoder consists of two recursively interconnected MAP symbol estimators between which a turbo code interleaver is arranged. The turbo decoding takes place iteratively in that the data signal to be decoded passes several times through a computing loop which contains the computing steps first symbol estimation—interleaving—second symbol estimation—deinterleaving.
An obstacle in the implementation of turbo codes is that turbo decoders require considerable signal processing and hardware expenditure due to the iterative signal processing and the two MAP symbol estimators.
For this reason turbo codes have hitherto only been used in systems less restricted in resources such as, for example, satellite communication systems. In the field of mobile radio, turbo codes are not yet used due to the the restrictions. However, the UMTS (Universal Mobile Telecommunications Systems) standard already includes the possibility of turbo coding.